Question

Which alkyl halide from the following pairs would you expect to react more rapidly by an SN2 mechanism? Explain your answer.
`(i) CH_(3)CH_(2)CH_(2)CH_(2)Br or CH_(3)CH_(2)underset(Br)underset(|)C HCH_(3) , (ii) CH_(3)CH_(2)underset(Br)underset(|)CHCH_(3) or H_(3)C-underset(CH_(3))underset(|) overset(CH_(3))overset(|)C-Br`
(iii)` CH_(3)underset(CH_(3))underset(|)CHCH_(2)CH_(2)Br or CH_(3)CH_(2)underset(CH_(3))underset(|)CHCH_(2)Br`

Answer 1

(i) `CH_(3)CH_(2)CH_(2)CH_(2)Br " "` Being primary halide, there won’t be anysteric hindrance.
(ii) `CH_(3)CH_(2)underset(Br)underset(|)CHCH_(3) " "` Secondary halide reacts faster than tertiary halide.
(iii) `CH_(3)underset(CH_(3))underset(|) CHCH_(2)CH_(2)Br " "` The presence of methyl group closer tothe halide group will increase the steric hindrance and decrease the rate.